Internal combustion engines, including diesel engines, gasoline engines, natural gas engines, and other engines known in the art exhaust a complex mixture of air pollutants. The air pollutants may be composed of gaseous compounds, which may include nitrogen oxides (NOx), and solid particulate matter, also known as soot. Due to increased attention on the environment, exhaust emission standards have become more stringent, and the amount of gaseous compounds emitted to the atmosphere from an engine may be regulated depending on the type of engine, size of engine, and/or class of engine.
One method that has been implemented by engine manufacturers to comply with the regulation of engine emissions is exhaust gas recirculation (EGR). EGR systems recirculate the exhaust gas byproducts into the intake air supply of the internal combustion engine. The exhaust gas, which is directed to the engine's cylinders, reduces the concentration of oxygen within the cylinders, which in turn lowers the combustion temperature within the cylinder. The lowered combustion temperature can slow the chemical reaction of the combustion process and, thereby, decrease the formation of NOx.
In order to avoid damage to the engine, particulates and corrosives formed during the combustion cycle must be removed before exhaust gas is reintroduced to the engine. In many EGR applications, the exhaust gas is directed through a diesel particulate filter, or diesel particulate trap, before reintroduction into the engine's air system. However, the diesel particulate filter, over time, can become worn or damaged and allow particulates and corrosive substances to be recirculated into the engine, adversely affecting performance and durability of the internal combustion engine and components of the EGR system.
As disclosed in U.S. Pat. No. 7,159,393 (the '393 patent), issued to Blomquist on Mar. 3, 2003, a second particulate filter may be used to remove particulates that pass though a first filter. Specifically, the '393 patent discloses an exhaust gas regenerator and particulate capture system that includes a first particulate filter and a second particulate filter disposed in series. The first particulate filter is located downstream of an engine and upstream of an EGR system. The first filter is capable of catching particulates typical of the combustion process, so that exhaust gases are sufficiently filtered for recirculation back into the engine. The second filter may be located within the EGR system, downstream of a cooler and may have a filtering efficiency equal to or lower than that of the first filter. Under normal conditions the second filter will not contribute to the catching of particulate constituents of the circulated exhaust gas, therefore it remains unclogged. However, if the first filter is malfunctioning and allows unfiltered or insufficiently filtered exhaust gases to pass though, the second filter will contribute to reducing the content of particulate constituents of the recirculated exhaust gases.
Although the exhaust gas regenerator/particulate capture system of the '393 patent may protect the engine from harmful particulate matter, it may leave the engine unprotected from damage caused by harmful corrosives such as sulfuric acid that form within the EGR system, as a result of the particulate matter. For example, when exhaust gas passes through an ERG cooler, sulfuric acid may condense on the walls of the cooler and downstream passages, thus exposing downstream EGR and engine components to acidic attack. Sulfuric acid is particularly likely to form when a fuel other than ultra-low sulfur fuel is used. Consequently, when an operator fuels the engine with fuel having a higher sulfur content, costly damages may result. The filter of the '393 patent may do little to neutralize or remove the sulfuric acid from the exhaust flow and may, itself, be subject to sulfuric acid attack.
The disclosed exhaust treatment system is directed to overcoming one or more of the problems set forth above.